18 OSTEOLOGY OF PTERANODON. 



equal and the oval articular cup and anterior articulating facets resemble those of the 

 last cervical. The transverse and capitular processes are remarkably large, and are com- 

 pletely fused with the stout ribs which they support. Posteriorly, the centrum of the 

 first dorsal vertebra is so perfectly coossified with that of the second dorsal as to be 

 inseparable and almost indistinguishable from it. The postexapophyses of the first 

 dorsal and the preexapophyses of the second are only vestigial. The centrum of the 

 second dorsal vertebra is narrower than the first centrum. The transverse and capitular 

 processes are much lighter, as are also the anchylosed ribs which they support. Since 

 the sutures are effaced, it has become impossible to determine accurately where the 

 capitula proper of the ribs begin and the corresponding processes end. This is also true 

 of the preceding vertebra and its ribs. In the second dorsal, however, the capitular 

 processes are given off from the centrum at a slightly higher level. The third centrum 

 is united to the second without the aid of exapophyses. The slender anchylosed rib 

 which it supports is still of the double-headed form, but the capitulum is situated so 

 high up on the centrum that the foramen enclosed by the two branches of the rib is 

 greatly reduced in size. The centra of the five succeeding vertebrae, the fourth, fifth, 

 sixth, seventh, and eighth of the notarium, progressively diminish in breadth until they 

 become nearly circular in section, being constricted in the middle and expanded at the 

 ends. Toward the end of the series, the fusion of the centra is possibly a little less 

 complete, and in one specimen, No. 2699, the fifth and sixth vertebras were found slightly 

 displaced, the exposed ends showing anteriorly a cup and posteriorly a reciprocal convex 

 articular surface. The zygapophyses, too, which are completely fused anteriorly, appear 

 less thoroughly united toward the rear end of the notarium. In the posterior vertebrae 

 of the series, the transverse processes are a little longer and are given oft* higher up on 

 the neural arches ; in fact, they originate considerably above the level of the neural 

 canal. Both the transverse processes and the neural spines are slighter than those of 

 the anterior dorsals. No ribs are in place behind the third pair, but the indications are 

 that all the posterior dorsal vertebrae carried slender single-headed ribs. With the no- 

 tarium of No. 2692, one free dorsal vertebra was found which is similar in form and size 

 to the last notarial, and may be accordingly regarded as the ninth of the dorsal series. 

 In another and larger example of Pteranodon sp., No. 2616 ; four dorsals similar to the 

 free dorsal of No. 2692 were found in close contact with each other, though not in 

 perfect articulation (Plate XIII, figure 1 ; also Plate VI, figures 17-20). Like all the 

 separate precaudal vertebrae, these four free dorsals are proccelous. Judging from the 

 character of the articulating surfaces, both central and zygapophysial, of the three an- 

 terior vertebrae of this series, and from the condition of the distal end of their trans- 

 verse processes and neural spines, which ^are not modified for attachment with either 

 notarium or pelvis, they are evidently free vertebrae. Front and rear views of the first 

 vertebra of this series are shown in Plate VI, figures 21 and 22. These same figures 

 would equally well portray the characters of the free dorsal found with No. 2692. The 

 fourth vertebra differs from the three preceding in having some further osseous develop- 

 ment at the extremity of its neural spine. This may be the vestige of a median longi- 

 tudinal band uniting the neural spines of the sacral vertebrae. Unfortunately, the anterior 

 portion of the sacrum of No. 2616 is not preserved, so that it is impossible to determine 

 to what extent the vertebra in question may have been connected with the sacral series. 

 Other examples of the genus offer clearer evidence upon the number of free dorsals and 

 are briefly described. 



